The present invention relates to a reduced electrode electrocardiography system. In particular the invention relates to a system for the synthesis of a standard twelve-lead electrocardiograph or similar, from measurements using fewer leads.
The electrocardiograph (or ECG) is one of the most important non-invasive diagnostic tools available to the cardiologist. During the development of electrocardiography, there has evolved a set of standard methods of obtaining an ECG from a subject. These standard methods include the “12 Lead ECG”, the “9 lead ECG”, the “15 lead ECG” and the “vectorcardiogram”.
The 12 lead ECG method is by far the most common of these methods and is thus often referred to as the “standard 12 lead ECG” or even just the “standard ECG”. Twelve “leads” (or signals) are obtained from a subject using ten electrodes placed on their skin, placed on standardised locations around the body. Each electrode is connected to a signal processing apparatus via a respective wire (or “lead”). The term “lead” commonly refers to either a physical wire to an electrode or to an ECG signal itself. To avoid confusion here, the use of the word “lead” will always refer to an ECG signal, never to electrode wiring.
The standard 12 lead ECG is divided into two sets: the limb leads I, II, III, aVR, aVL and aVF and the precordial leads V1, V2, V3, V4, V5 and V6.
Unfortunately, the application of the 12 lead ECG is problematic: The use of ten electrodes plus associated wiring often causes discomfort to the subject, even on short timescales. The placement of ten electrodes can take considerable time, particularly if carried out by a non-specialist. Additionally, the amount of electrode wiring can impede the clinician when performing other procedures on the subject, while the cost of the procedure is increased when using a larger number of electrodes.
In the case of “ambulatory” recording, the situation is much worse. During ambulatory recording, measurements are made while the subject is free to move around, for example walking, running etc. The limb electrodes can thus be subject to severe motion and muscular artefacts that corrupt the desired ECG signals. Electrode wiring connected to the arms and legs also restricts the movement of the subject and the clothing that they can wear. The use of a large number of wires increases the opportunity for one or more of these wires to become caught or snagged while the subject is moving, possibly causing electrode detachment or denigration of the electrode/skin contact. In addition, electrode/skin contacts often become irritable if worn for a significant length of time, a problem that is clearly exacerbated by the number of electrodes worn.
Several reduced lead sets methods have been proposed to provide an approximation of a standard ECG using fewer electrodes placed at carefully defined electrode locations. For example U.S. Pat. Nos. 4,106,495, 4,318,412, 4,850,370, 5,058,598, 6,052,615 and 6,119,035 each use methods involving linear transformations between the leads recorded and the desired lead set, typically either the standard 12 lead ECG or the vector cardiogram. All of these systems reduce the difficulty, setup time, discomfort and unit cost of an ECG recording. This is balanced against the inevitable differences between the true standard ECG recording and the synthesised alternative. The use of a reduced set of electrodes is also of significant worth if the ECG must be sent over a communications network, where bandwidth availability may have to be taken into account.
A problem with known reduced electrode sets is that the transformations required to synthesise the desired lead(s) are not constant for different subjects. Known systems have used either fixed transformations (determined over a large population of subjects) to approximate the transformations required, or subject-specific transformations that are calculated per subject, the latter calculation requiring both the desired lead(s) and the reduced lead set to be measured on the subject in question as a preliminary step.
A further problem is that the synthesis transformations are not constant even for the same subject under a variety of body postures. This is because as the subject's body posture changes, so too does their body shape. If the subject maintains the same single posture for which the lead transformations were defined, for example the classic “resting ECG” position where the subject is reclined on their back, then this posture effect will not occur. However an ambulatory subject will exhibit several different body postures and thus posture induced changes in the synthesis transformations become relevant.
Known reduced-set solutions can also be error prone because of lack of familiarity with the non-standard ECG positions they use, and for the same reason, it can be hard to verify accuracy of readings.